1. Field of the Invention
The present invention relates to electro-optic waveguide devices, in particular to electro-optic modulators, to a method of fabricating travelling-wave RF electro-optic modulators and devices produced thereby.
2. Discussion of Prior Art
Conventional wafer based waveguides provide a convenient means for guiding light within a semiconductor wafer. In such waveguides light is confined within a core layer by cladding layers, exhibiting a lower refractive index than the core material, arranged above and below the core layer. Light is confined at the edges of the waveguide by the core/air interface.
For example, such waveguides can be fabricated from III–V semiconductors such as lattice matched GaAs/AlGaAs. In this example the refractive index of the general material AlxGa(1-x)As depends on the aluminium mole fraction (x). Propagation of light may be confined primarily to the plane of the semiconductor wafer by growing epitaxial layers of AlxGa(1-x)As with different values of the aluminium mole fraction x. Propagation of light may be further confined to a narrow waveguide by etching the requisite waveguide pattern into the AlxGa(1-x)As layers.
It is well known that the optical properties of the waveguide may be altered by applying an electric field across the waveguide. For example the refractive index of the waveguide core (and the upper cladding layer) may be altered via the linear electro-optic effect (Pockels effect), which in turn may be used to alter the phase of light passing through the waveguide.
In practice, the core and upper cladding layers are undoped and behave effectively as insulators. The lower cladding layers are usually doped to provide a conducting layer below the waveguide. The substrate is usually undoped and also behaves effectively as an insulator. By placing an electrode on the top of the waveguide, and a second electrode in contact with the doped lower cladding layer, an electric field can be applied across the guiding region by applying a voltage between the upper and lower electrodes. Accordingly, the refractive index of the waveguide core, and hence the phase of light passing through the waveguide, may be modulated electrically.
This effect provides the basis for electro-optic waveguide interferometers in which light can be switched from one output to another by applying an electric field across the waveguide.
The linear electro-optic effect is a very fast effect, and the switching speed of the waveguide interferometer depends mainly on how quickly the voltage can be applied to the electrodes. For a simple electro-optic waveguide interferometer the switching speed will be limited by the capacitance of the electrode, or in practice, by the capacitance of the bond pad used to connect a wire to the electrode.
The linear electro-optic effect is however a weak effect, necessitating long electrodes (of the order of tens of millimeters in length) within the device in order to keep the drive voltage at acceptable levels. The length of the electrode can cause complications if the device is to be operated at very high switching speeds (for example where the interferometer is switched at radio frequencies (RF) of the order of 50 GHz) since the length of the electrode is long compared to the wavelength of the RF drive signal (for example, a 50 GHz drive signal has a wavelength of 6 mm in air). This means the transit time of the light under the electrode would correspond to a few RF cycle periods. The light would be phase shifted in one direction by the positive half-cycles of the RF drive signal, and then phase shifted in the opposite direction by the negative half-cycles, before it has had time to pass under the full length of the electrode. The total phase shift would be small, if not zero, and so the device would not be very efficient.
To overcome this effect, the RF wave must be made to travel along the device in the same direction, and at the same speed, as the light in the waveguides. In this manner the same part of the RF wave always acts on the same part of the light beam, and the required optical phase shift grows continuously as the light propagates along the device. This type of device is called a “travelling-wave” electro-optic waveguide modulator.
One of the main difficulties in making this type of travelling-wave electro-optic modulator is that the epitaxy needed to apply the RF drive signal to the waveguide has to have conducting lower cladding layers in order to confine the applied field strongly in the core and upper cladding layers only.
Due to the high frequency of the drive signal, coplanar stripline is employed to route the RF drive signal to the modulator electrodes. If the coplanar stripline (CPS) were placed on top of the conducting layers of epitaxy (the doped lower cladding layer of the waveguide), the CPS line would be very lossy and the RF drive signal would propagate only a few millimeters at most before being completely attenuated by the conducting layers. Ideally, the CPS line should be placed on an insulating (or in semiconductor terms, semi-insulating) substrate to avoid severe losses. However, further processing would then be required to provide a conducting epitaxial layer beneath the waveguide in order to confine the applied field within the waveguide core and upper cladding layers.
An alternative method of reducing unwanted attenuation of the RF drive signal is to fabricate the device on a semi-insulating substrate and to etch a deep trench between the coplanar stripline primary electrodes and any conducting epitaxy employed underneath the waveguide core, thereby effectively isolating the two structures. In this configuration bridging electrodes are utilised to span the trench between the coplanar stripline and the waveguide [see for example “High-speed III–V semiconductor intensity modulators”, Walker R G, IEEE Journal of Quantum Electronics, 27: (3) 654–667 March 1991]. However, this isolation technique is complex and expensive due to low processing yields. The technique also results in a non-planar substrate making subsequent processing of the device more difficult.
It is an object of the present invention to provide an improved method for producing an electro-optic waveguide modulator and specifically for producing a travelling-wave electro-optic modulator.